Degradation and corrosion damage imposes an enormous cost throughout the world. In the United States alone, the annual cost of corrosion damage has been estimated to be equivalent to 4.2% of the gross national product (Martinez, L. J. Metals. 45:21 (1993)) (hereafter, Martinez, 1993). These large costs could be greatly reduced by better and wider use of corrosion protection techniques.
Microbes contribute significantly to degradation and corrosion damage. When surfaces, and particularly metals, are exposed to natural environments, they are rapidly colonized by aerobic bacteria present in the bulk liquid phase (Geesey, G. G., What is biocorrosion? Presented at the International workshop on industrial biofouling and biocorrosion, Stuttgart, Germany. Springer-Verlag, New York (1990)) (hereafter, Geesey, 1990). The upper layers of this biofilm are aerobic while the regions near the metal surface are anoxic due to the depletion of oxygen by the biofilm (Blenkinsopp, S. A. et al., Trends. Biotechnol. 9:138–143 (1991); Bryers, J. D. et al., Biotech. Prog. 3:57–67 (1987)). Sulfate-reducing bacteria (“SRB”) can colonize these anaerobic niches and thus contribute to corrosion even in an aerobic environment (Hamilton, W. A. Sulphate-reducing bacteria and their role in biocorrosion. Presented at the International workshop on industrial biofouling and biocorrosion, Stuttgart, Germany. Springer-Verlag (1990)) (hereafter, Hamilton, 1990).
SRB have been implicated in the deterioration of metals in a wide range of environments (Borenstein, S. W. Microbiologically influenced corrosion handbook. Woodhead Publishing Limited, Cambridge, England (1994) (hereafter, “Borenstein, 1994”); Hamilton, W. A. Ann. Rev. Microbiol. 39:195–217 (1985) (hereafter “Hamilton, 1985”); Hamilton, W. A. Trends. Biotechnol. 1:36–40 (1983); Hamilton, 1990). Pipelines and off-shore oil rigs in the oil and shipping industries (Hamilton, W. A. Trends. Biotechnol. 1:36–40 (1983)), cooling water recirculation systems in industrial systems (Borenstein, 1994; Miller, J. D. Metals, p. 150–201. In Rose, A. H. (ed.), Microbial Deterioration, Academic Press, New York (1981)) (hereafter, Miller, 1981), sewage treatment facilities and pipelines (Hamilton, 1985); Odom, J. M. ASM NEWS. 56:473–476 (1990)), jet fuel tanks in the aviation industry (Miller, 1981), and the power generation industry (Licina, G. J. Mater. Perform. 28:55–60 (1989)) (hereafter, Licina, 1989) have all been adversely affected by the growth and colonization of SRB. SRB can cause corrosion of a wide range of metals like low-grade carbon steels (e.g., Borshchevskii, A. M. et al., Prot. Metals. 30:313–316 (1994); Cheung, C. W. S. and Beech, I. B., Biofouling. 9:231–249 (1996) (hereafter, Cheung and Beech, 1996); Dubey, R. S. et al., Ind. J. Chem. Tech. 2:327–329 (1995); Gaylarde, C. C. Int. Biodet. Biodeg. 30:331–338 (1992)) (hereafter, Gaylarde, 1992); Lee et al., Biofouling 7:197–216 (1993); stainless steels, (Benbouzid-Rollet, N. et al., J. Appl. Bacteriol. 71:244–251 (1991); Mollica, A. Int. Biodet. Biodeg. 29:213–229 (1992); Newman, R. C. et al., ISIJ International. 3:201–209 (1991)); Oritz et al., Int. Biodet. 26:315–326 (1990)); and copper alloys (Licina, 1989; Wagner, P. and Little, B., Mater. Perform. 32:65–68 (1993)) (hereafter, Wagner and Little, 1993), all of which are frequently used in process, shipping, and power industries. SRB also contribute substantially to the degradation of nonmetallic portions of the world's infrastructure. SRB produce hydrogen sulfide, which is then metabolized by sulfur-oxidizing organisms such as Thiobacillus into sulfuric acid. Sulfuric acid degradation due to bacteria has been found to reduce dramatically, for example, the service life of concrete conduits in water systems. Corrosion damage due to SRB just of metals in the U.S. has been estimated to amount to some $4–6 billion annually (Beloglazov, S. M. et al., Prot. Met. USSR. 27:810–813 (1991)) (hereafter, Beloglazov, 1991).
Conventional corrosion inhibition strategies have included a modification in the pH, redox potential, and resistivity of the soil in which the equipment is to be installed (Iverson, W. P. Adv. Appl. Microbiol. 32:1–36 (1987)) (hereafter, Iverson, 1987), inorganic coatings, cathodic protection, and biocides (Jack, T. R. et al., Control in Industrial Settings, p. 265–292. In Barton, L. L. (ed.), Sulfate-reducing Bacteria. Plenum Press, New York (1995)) (hereafter, Jack et al., 1995) (the entirety of the Barton reference is hereby incorporated by reference). Inorganic protective coatings like paints and epoxies have been used extensively in the past; but, they are not permanent, and the cost of maintaining and replacing them is substantial (Jayaraman, A., et al., Appl. Microbiol. Biotechnol. 47:62–68 (1997) (hereafter, Jayaraman et al., 1997a); Martinez, 1993). With cathodic protection, the cathodic reaction is stimulated on the metal surface by coupling it to a sacrificial anode made of magnesium or zinc, or by supplying an impressed current from an external power supply through a corrosion-resistant anode. The galvanic or impressed current lowers the electrochemical potential everywhere on the metal surface so that metal cations do not form, and no dissolution occurs. ((Iverson, 1987); Little, B. J. et al., Mater. Perform. 32:16–20 (1993)). However, Wagner and Little (1993) report that the use of cathodic potentials up to −1074 mV were not able to prevent biofilm formation.
Biocides have also been used to retard the corrosion reaction in closed systems such as cooling towers and storage tanks (Iverson, 1987)) and are probably the most common method of combating biocorrosion (Boivin, J., Mater. Perform. 34:65–68 1995) (hereafter, Boivin, 1995); Brunt, K. D., Biocides for the oil industry, p. 201–207, In Hill, E. C., Shennan, J. L., Watkinson, R. J. (ed.), Microbial Problems in the Offshore Oil Industry, John Wiley and Sons, Chichester, England (1986); Cheung, C. W. S. et al., Biofouling 9:231–249 (1996)) (hereafter, Cheung, 1996). Saleh et al. (J. Appl. Bacteriol. 27:281–293 (1964)) (hereafter, Saleh et al., 1964) reviewed the use of nearly 200 compounds that are bactericidal or bacteriostatic against SRB. Oxidizing biocides like chlorine, chloramines, and chlorinating compounds are used in freshwater systems (Boivin, 1995, supra). Chlorine compounds are the most practical biocides; however, their activity depends on the pH of the water and the extent of light and temperature (Keevil, C. W. et al., Int. Biodet. 26:169–179 (1990)) (hereafter, Keevil et al., 1990), and they are not very effective against biofilm bacteria (Boivin, 1995, supra). Non-oxidizing biocides such as quartenary salts (Beloglazov, 1991), amine-type compounds, anthraquinones (Cooling III, F. B. et al., Appl. Environ. Microbiol. 62:2999–3004 (1996)) (hereafter, Cooling et al., 1996), and aldehydes (Boivin, 1995) are more stable and can be used in a variety of environments. Use of these biocides suffer from a number of serious drawbacks, including not only cost of the biocides themselves but also the environmental cost of releasing into the water supply large quantities of inorganic compounds.
A further problem is imposed by the organization of the biofilm on the material surface. The glycocalyx (Brown, M. L. et al., Appl. Environ. Microbiol. 61:187–193 (1995); Hoyle, B. D. et al., J Antimicrob. Chemother. 26:1–6 (1990); Suci, P. A. et al., Antimicrob. Agents Chemother. 38:2125–2133 (1994)), phenotypical changes which occur in the biofilm, such as the expression of the algC gene in P. aeruginosa (Costerton, W. J. et al., Ann. Rev. Microbiol. 49:711–745 (1995)) (hereafter, Costerton, 1995), and the effect of surface chemistry on the metabolic state of the biofilm (Keevil et al., 1990) may all serve to increase the resistance of organisms in a biofilm to antimicrobial agents beyond that observed with planktonic bacteria (Brown, M. R. W. et al., J. Appl. Bacteriol. Symp. Suppl. 74:87S-97S (1993)). A combination of an organic film-corrosion inhibitor, a polyacrylate/phosphonate, and two biocides has been used successfully to control corrosion in a cooling water system (Iverson, supra). However, SRB are inherently resistant to a wide range of antimicrobials (Saleh et al., 1964, supra), and the harsh anaerobic environment (created by the corrosion products) in which the SRB thrive also reduces the efficiency of the antimicrobials (Cheung, 1996; Iverson, supra). Once SRB are firmly established in their niche, it is difficult to eliminate them from a system without disassembling it (Boivin, 1995, supra).
Another strategy to control microbially induced corrosion is to suppress the growth of the most harmful microorganisms by manipulating the nutrient availability and thereby create a more benign biofilm (Jack et al., 1995). Recently, Jansen and Kohnen (J. Ind. Microb., 15:391–396 (1995)) reported the reduction in the adherence of Staphylococcus epidermis KH6 to surfaces by modifying the polymer surface by ionic bonding of silver ions to the surface and suggested the development of antimicrobial polymers to prevent bacterial adherence. Wood, P., et al. (1996) (Appl. Environ. Microbiol. 62:2598–2602) reported the generation of potassium monopersulfate and hydrogen peroxide at the surface by catalysis increased the activity of these biocides 150-fold towards a P. aeruginosa biofilm. This method relied on permeating a plastic with the necessary chemical agents, and would require widespread, substantial, and costly changes in manufacturing techniques to implement.
Finally, work by others suggested (Pedersen and Hermansson, Biofouling, 1:313–322 (1989), and Biofouling 3: 1–11 (1991)), and our own work has recently confirmed (Jayaraman et al., 1997a and Jayaraman et al., J. Ind. Microb. 18:396–401 (1997) (hereafter, Jayaraman et al. 1997b), that aerobic bacteria in a biofilm can inhibit electrochemical corrosion of metal by two to forty fold, possibly due in part to the fact that respiring bacteria in a biofilm on a metal use some of the oxygen which would otherwise be available to oxidize that metal. As noted above, however, this reduction of oxygen level also creates an opportunity for SRB, which are anaerobic, to colonize the metal. Thus, in practice, the effectiveness of biofilms as a means of inhibiting electrochemical corrosion is reduced by the consequent enhancement of the rate of SRB-related corrosion.
What is needed in the art is an effective and less expensive means to prevent or inhibit SRB-caused corrosion or degradation, with lessened release of toxic agents into the environment. The present invention provides these and other advantages.